Automatically accept contributions if they have nothing to add

The reason behind this is that maintainers tend to manually accept the
contribution (which I can understand). So we just do it automatically if there
is a high chance that the contribution was just committed.

Note that there are certain situations where a contribution can be marked as
accepted by accident but that should be a rare case such as multiple
contributions all providing the same strings etc.
Make the checker methods API cleaner
Make checker output more Jenkins friendly

As we execute this via Jenkins now, there is no need to display errors to
stdout (which would make them displayed twice in the Jenkins console output).
MDLSITE-2541 Check for contributions pending in the reviewed state